The invention relates to a method and apparatus for the fabrication of a biaxially stretched flat film generally from thermo-plastic materials such as polyolefins, and particularly from polypropylene. More particularly, the invention can be used as a process for improving the properties of a film of thermoplastic material by generally stretching a flat film of such material in two directions, and particularly by heating the film to an elevated temperature below its melting point (in a range, for example, of 50.degree. C. to 170.degree. C.), stretching the film first partially in its longitudinal direction, thereafter, at the same elevated temperature, stretching the film simultaneously in both directions (longitudinally and transversely), thereby simultaneously reaching the remainder of the stretching capacity of the film in the longitudinal direction and its total stretching capacity in the transverse direction, and cooling the stretched film to freeze (or make permanent) the film's orientation produced by stretching. The partial stretching in the first step in the longitudinal direction is preferably 25% to 50% of the total stretching capacity of the film in the longitudinal direction.
It is common practice in manufacturing biaxially stretched films to stretch the film first in one step in a device containing rollers arranged in a series in the longitudinal direction and then in another step to stretch the film in the transverse direction whereby the edges of the flat film are engaged by gripping means which are moved in a diverging relationship (e.g. as disclosed in U.S. Pat. No. 2,702,406 to E. G. Reed). However, particularly in the case of crystalline thermoplastic materials, such as, e.g. polyolefins, stretching in the longitudinal direction cannot be effected up to the optimum longitudinal stretching capacity of the finished film because the film obtained after the longitudinal stretching operation has such a great tendency to split that it cannot thereafter be stretched in the transverse direction. In the case of polypropylene films equal and maximum physical properties are achieved in both directions (longitudinally and transversely) if the stretching ratio in each of the directions is approximately 1:7, in other words, 1:7 in the longitudinal direction and 1:7 in the transverse direction, thereby resulting in an increase of approximately 49 times of the original or unstretched film area. Sometimes polypropylene films, for a given use they are to be applied to, are required to have a stretching ratio in longitudinal direction of up to 1:11, the total amount of stretching of the overall area of the film being about 1:50, the stretching ratio in the transverse direction, therefore, being 1:50/11.
Accordingly, for the purposes herein, when referring to "stretching ratios", we can refer to one which is "unidirectional"--namely, either longitudinal or transverse, or we can refer to one that is "bidirectional"--namely, one that is the mathematical product of the amount of stretch in the longitudinal and transverse directions.
Other biaxially stretched films are entirely simultaneously stretched in the longitudinal and transverse directions. In such operations, flat films are laterally constrained and guided by means of various gripping devices. An increase of stretching in the longitudinal direction leads to an increase in the spacing between the gripping devices, between which devices the film will show a severe deformation in the form of a "neck-in". The bent-out edge sections of the film will display an uneven distribution of tensile strength values, so that they have to be cut off. The resultant loss in material can amount to in excess of 30% of the final width of the finished film.
A device for biaxially stretching films is disclosed in German Patent Specification No. 1,088,705. The disclosed gripping devices have their speed increased by spindles having a progressive pitch. Another device for biaxially stretching films is disclosed in U.S. Pat. No. 3,890,421, wherein each gripping device disclosed is moved independently from the others by a linear motor.
Such known devices for effecting a simultaneous biaxial stretching of film in the longitudinal and transverse directions have the disadvantage that as the degree of longitudinal stretching increases, there results an undesirable increase in the spacing between the gripping devices, as described above, and therefore such devices cannot be used when a high stretch degree of 1:5 to 1:10 in the longitudinal direction is required.